server: MTU probe (MTU_PROBE/MTU_ACK) — path-MTU / black-hole measurement

Server ACKs each DF-flagged probe with a tiny MTU_ACK carrying the size it
received; the client binary-searches the path MTU. Non-amplifying by
construction. Tested.

Also records: v0.3.2 (http-echo + tls-reference) verified live on fmr, and
the finding that upstream trains are already observable via the
observations API (dedicated TRAIN_REPORT deferred — needs an
anti-amplification grant + columnar encoding).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/internal/dataplane/udp.go

@@ -29,6 +29,8 @@ const (
 	TypeEchoResp    = 0x02
 	TypeTimesyncReq = 0x07
 	TypeTimesyncRsp = 0x08
+	TypeMtuProbe    = 0x09
+	TypeMtuAck      = 0x0A
 	TypeDelayedEcho = 0x0B
 )
 
@@ -132,11 +134,24 @@ func (s *Server) handle(conn *net.UDPConn, raddr netip.AddrPort, pkt []byte, tRx
 		s.echoResp(conn, raddr, sess, pkt, seq, tRxNs)
 	case TypeTimesyncReq:
 		s.timesyncResp(conn, raddr, sess, pkt, seq, tRxNs)
+	case TypeMtuProbe:
+		s.mtuAck(conn, raddr, sess, seq, len(pkt))
 	default:
 		slog.Debug("unhandled data-plane type", "type", typ)
 	}
 }
 
+// mtuAck replies to an MTU_PROBE with a small MTU_ACK carrying the total
+// datagram size the server actually received (spec §3.2). The client sends
+// DF-flagged probes of increasing size and binary-searches the path MTU / a
+// black hole from which sizes stop being acknowledged. The ACK is tiny, so it
+// can never amplify regardless of probe size.
+func (s *Server) mtuAck(conn *net.UDPConn, raddr netip.AddrPort, sess *session.Session, seq uint32, received int) {
+	var payload [4]byte
+	binary.BigEndian.PutUint32(payload[:], uint32(received))
+	s.send(conn, raddr, sess, TypeMtuAck, seq, payload[:])
+}
+
 // Observation block (spec §3.3), fixed 40 bytes appended to the RESP header:
 //   0  8  t_rx_ns (server clock, process epoch)
 //   8  8  t_tx_ns